7 Choices?! Strange Creature Picks Its Sex at Random

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With seven different sexes to choose from, the single-celled
organism Tetrahymena thermophila determines its
biological mating type in a game of molecular chance, new
research finds.

Tetrahymena are oval-shaped protozoa that live in
freshwater. These microscopic organisms come in seven different
"sexes," or mating types. Any Tetrahymena sex can mate with any other mating type
except its own.

Even more intriguing to biologists is that it doesn't matter what
mating types two Tetrahymena parents are. In fact, their
offspring can be any one of the seven. That observation had some
strange genetic implications, as parents typically would pass on
their own mating-type genes to their progeny.

But even though scientists have known about
Tetrahymena's seven mating varieties for 60 years, they
have only now discovered how the individuals select their type.

"Finally, we had the resources to get at the molecular basis of
it ― to actually discover the mating-type genes, what their
sequence is, and how it is that the cells have the potential for
many mating types and only end up expressing one ― a random one,"
study researcher Eduardo Orias, a biologist at the University of
California, Santa Barbara, told LiveScience.

By peering into the Tetrahymena genome, the researchers
found the equivalent of a cellular roulette wheel. The organisms
have two nuclei apiece. One, the somatic nucleus, contains the
DNA that does the daily work of the cell. The other, the germline
nucleus, acts like the cells in the ovaries or testes of humans.
The DNA in the germline passes along traits to offspring.
[ Sex Quiz:
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When two Tetrahymena fuse in their version of single-cell
sex, they produce a gamete nucleus, which is the protozoan
equivalent of a fertilized egg in humans. This fertilization
nucleus starts making copies of itself, some of which are
destined to become germline nuclei and some of which are somatic.

It is during this step that the mating type is chosen, the
researchers found. Each germline nucleus holds an array of
incomplete gene pairs ― one for each of the organism's seven
sexes. The cell joins and completes one of these gene pairs
randomly, thus setting the cell's mating type. The rest of the
incomplete gene pairs are thrown out, said the report, released
by Orias and his colleagues today (March 26) in the journal
PLOS Biology.

"We had no idea what a beautifully organized system this turned
out to be," Orias said. "It's very modularly organized and very
symmetrical in some ways and very ― to us ― aesthetically
exciting."

Having seven mating types, instead of only two, may make it more
likely for Tetrahymena to run into a cell they can
reproduce with when they meet and greet in a pond, Orias said.